Electric valve circuits



Apfil 22, 1947. BEDFORD 2,419,465

ELECTRIC. VALVE cmbums Fil ed June 12, 1944 Inventor! Burnice D. Bedf'ord,

b 1 6/7 HiZ Kr,

tor neg.

Patented Apr. 22, 1947 2,419,465 ELECTRIC VALVE CIRCUITS Burnice D. Bedford,

to General Electric Com New York Application June 12, 1944,

12 Claims. (01. 315-262) My invention relates to electric valve circuits and more particularly to control or excitation circuits for electric valves of the type employing an ionizing medium, such as a gas or a vapor.

In the control of electric valve apparatus including an ionizable medium such as a gas or vapor capable of supporting an arc discharge, one critical period in the operating cycle is the deionization period. This is particularly true when the valve apparatus is used as an inverter to convert direct current to alternating current. Grids have been suggested and used heretofore in both thyratrons and ignitrons to obtain accurate starting and rapid deionization in inverters. However, in the ignitron type of valve where the cathode spot is intended to be formed every half cycle an undesirable situation arises in the event the ignitor fails to fire or for some reason the cathode spot fails to form. If, under these conditions, the grid is made positive the residual ionization in the valve may form a cathode spot by bombardment on the side walls and other equally undesirable locations within the valve envelope.

It is an object of my invention to provide new and improved excitation and control circuits for electric valves.

It is another object of my invention to provide new and improved excitation and control circuits for a gas-discharge valve with a pool type cathode in which an ignition means is employed to control the starting of unidirectional current flow in each operating cycle.

It is a further object of my invention to provide new and improved excitation and control circuits for the ignitron-type of valve which will overcome the difficulties above set forth.

In accordance with the illustrated embodiment of my invention, I employ a modifying or coupling means between a holding or auxiliary anode circuit. of the electric valve and the grid excitation circuit of the valve whereby the excitation circuit cannot be energized suiliciently to effect conduction in the valve in the absence of proper electron emission from the cathode such as the formation of a cathode spot in the regular and intended manner.

My invention will be better understood from the following description taken in connection with the. accompanying drawing and its scope will be pointed out'in the appended claims.

. The single figure of the accompanying drawing illustrates, one embodiment of my invention as. applied to. an electric valve conversion system.

In the drawin s I have illustrated one form Schenectady,

- each of the valves N. Y., assignor pany, a corporation of Serial No. 539,941

of my invention as embodied in an electric valve conversion system comprising two valve groups I and 2 which may be taken as representative of a system adapted for converting from direct cur-' rent to alternating current, or vice versa, such as might be utilized at one end of a high voltage direct current transmission line or dual conversion type frequency converter unit. There are six valves in each group arranged in a threephase double-way circuit. Valve group I isconnected through a transformer 3 to a circuit 4 which may be taken to represent either a supply circuit or a receiving circuit. Valve group 2 is connected similarly to valve group I through a transformer 5 to the circuit 4.

Since the particular details of the arrangement of the valves form no part of my present invention, it is believed sufiicient for an understanding of the invention to describe in detail the excitation and control circuits for one valve pair of valve group I. The valves of this pair are rendered conductive electrical degrees apart and are identified as 6 and l. The type of valve selected for purposes of illustration is known in the art as a pentode ignitron by reason of the use of three grids in addition to the other two principal electrodes. Valves 6 and 1 are each provided with an anode 8, a first electrode or grid 9 which acts as an intermediate anode, a control electrode or grid in which determines the time of starting and reduces the deionization period at the end of conduction, and an outer control electrode or grid H which acts in a dual capacity to pick up the are at the beginning of conduction and to shield the other grids and anode structure from residual ionization after conduction. In addition, 6 and l is provided with a mercury pool cathode and two ignitors l3 and it although only one need be used at one time. Above the surface of the mercury are arranged auxiliary anodes or holding anodes l5 and IS. The auxiliary anode I5 may function primarily to maintain the cathode spot. for low values of anode current and the auxiliary anode i6 is utilized in accordance with my invention to prevent the possible uncontrolled formation of cathode spots through it coupling with the grid excitation circuit about tobe described.

The power supply for both the ignitor circuits and the grid circuits may conveniently be obtained, as illustrated, from a single auxiliary transformer I! through a multiple-phase shifting device 18 in order to control the respective pairs of valves. A satisfactory type of ignitor circuit is of the so-called magnetic type and a particularly satisfactory species of this type is the illustrated form which has been described and claimed in application Serial No. 413,232, of A. H. Mittag, filed October 1, 1941, and assigned to the assignee of the present application. The ignitor circuit I9 is energized from a circuit 20 of the phase shifter I8 of appropriate phase and transmits unidirectional impulses of current of peaked wave form to the ignitor members I 3 and I4 of the valves 6 and 1 and causes a cathode spot to be established in these valves alternately during intervals of time displaced substantially 180 electrical degrees. The circuit I9 is connected to the respective ignitor circuits through an insulating transformer 2I having a primary winding 22 and a pair of secondary windings 23 and 24. The insulting transformer is used to transform the ignitor peaks up to the high potential level of the valves. The primary Winding 22 is energized from the circuit 29 through a nonlinear reactance' or firing reactor 25 and a shunt connected capacitance 26, which elements in combination cause a peak of voltage to be generated across the secondary windings of transformer 2I. These impulses of voltage occur twice during each cycle of voltage of circuit 20 at times determined by the saturation of the firing reactor 25 and are transmitted to the respective ignitor circuits through the secondary windings 23 and 24. Since a pair of ignitors are used in each valve, one terminal of the winding 24 is connected to the oathode circuit of valve 6 and the other terminal is connected to the midtap of a current dividing reactor 21 having its outer terminals connected to ignitors I3 and I4 through unidirectional con ducting devices 28 and 29. The secondary winding 23 is similarly connected to the ignitors I3 and I4 of valve 8 through a midtapped current dividing reactor 39 and unidirectional conducting devices 28 and 29. The circuit I9 also includes a linear reactance 3i connected between the circuit 20 and the capacitor 26 which serves to prevent discharge of the capacitance to the supply circuit upon saturation of the firing reactor 25, and which also serves to limit the amount of current derived from the supply circuit at the time capacitance 26 discharges through transformer winding 22. A further reactor 32 is connected in shunt to the circuit input terminals of the firing reactor 25 and firing capacitor 26. This reactor is designed to operate somewhat above saturation and is preferably provided with a core material to saturate sharply so that it will tend to hold substantially constant voltage over wide limits of variation in the supply voltage. The circuit I9 is also provided with an additional shunt connected compensating circuit comprising a capacitance 33 connected in series with a linear reactor 34 across the supply circuit and a series connected linear reactance 35 for maintaining'the voltage applied to the firing capacitance 26 substantially constant for a predetermined range of phase displacements of the voltage of the firing circuit with respect to the supply circuit voltage.

Since the firing circuit I9 furnishes current to the ignitors for only a relatively few degrees after the ignitor fires, a holding anode I is utilized to maintain the cathode spot for substantially the remainder of the half cycle of positive anode voltage and is conveniently energized from the source of excitation for the grid excitation circuit 36. The circuit 36 includes a transformer 37, which may be referred to as the grid excitation transformer. Transformer 3! is provided with a primary winding 38 and a pair of insulating secondary windings 39 and 40 which isolate the grid circuit from ground potential. The holding anode I 5 is energized from a transformer 4| having a primary winding 42 connected to be energized from the secondary winding 39 of the grid excitation transformer 31. The secondary winding 43 of the holding anode transformer 41 has one terminal thereof connected to the oathode of valve 6 and the other terminal connected to holding anode I5 through a current limiting resistor 44 and a unidirectional conducting device 45.

Negative bias for the control grid I0 and shield grid I I is supplied by a bi-phase, double-way rectifier 46 which is energized through a transformer 41 also connected to be energized from the secondary winding 39 of the grid transformer. A capacitor 48 and resistance network 49 comprising resistors 49' and 49" serve to regulate the grid bias voltage. The resistors 49' and 49" preferably should have a nonlinear volt-ampere characteristic, such as the resistance material having the trade-mark Thyrite, which is disclosed and claimed in United States Letters Patent No. 1,822,742, granted September 8, 1931, upon an application of Karl B. McEachron.

In the event the valve groups I and 2 are operated as phase commutated inverters, accurate firing is essential, since a small error will mean a much greater change in the deionization angle. In order to obtain the desired accuracy of firing a peaking transformer 50 is utilized to introduce a positive pulse of voltage in circuit with control and shield grids I0 and II. The peaker voltage must necessarily rise higher than the negative bias voltage and the rise should be so rapid that negligible error is introduced if one valve requires a; greater positive grid voltage than another. The peaker transformer 50 is provided with a primary winding 5|, which is connected to be energized from the grid transformer secondary winding 39, and a secondary winding 52. In accordance with my invention, I make the firing of the grid dependent upon the controlled formation of a cathode spot. One means of accomplishing this interlocking or coupling which has shown satisfactory operation in practice is to connect the secondary winding 52 of the peaker transformer 59 in a circuit through the auxiliary or holding anode I6 and provide in series relation with this circuit a coupling impedance or, as illustrated, a transformer 53. The transformer 53 is provided with a primary winding 54 connected in series relation with the auxiliary anode I6 and a secondary winding 55. Thus when this holding anode circuit is completed by the arc path through auxiliary anode IS, the peaker impulse will appear in the winding 55 and be impressed upon the grids I0 and I I. A resistor 56 having a nonlinear volt-ampere characteristic, such as that specified for resistances 49' and 49", may be connected across the winding 54 as a voltage pro-- tective means. The winding 54 is connected to the auxiliary anode I6 through an adjustable linear resistance 51 and a unidirectional conductive device 57. The resistor 51 prevents undue variations in the duration of the peaks irrespective of variations in the impedances of the circuits of auxiliary-anode I 6 and grids ID and II. The resistor 51 is made adjustable to adjust the duration of the peaking impulse. The secondary winding 55 is provided with an intermediate tap 58 which is connected through a current limiting and adjusting resistor 59 to the The'path for the peaking impulse impressed upon grids lll'and H may be traced from the cathode 12 of valve :6 through the upper resistor '45, through grid :firing :resistor 60, through transformer secondary winding 55 to grids H! and 11.

It will be noted that the peaking impulses throughthe primary winding 54 of the coupling transformer 53 are unidirectional impulses which tend tocause an accumulative direct current saturation of the core of this transformer. In order to :avoid :this accumulative saturation, I provide a circuit to circulate a current in opposition to the magnetizing current of the transformer '53 which :is :derived from the current in the circuit of the secondary winding :55. This circuit includes part of the principal grid :firin'g circuit from the junction of resistors as and d8", through resistor 60, through the lower portion of secondary winding '55, out of the tap Eli, through the linear resistor 59 to the most negative point of the voltage divider 59 which is the lower terminal of resistor 49". This circuit becomes effective to circulate the reverse current through winding 55 upon the termination of the peak of current in primary winding '54, Since the auxiliary anode I5 is energized :by apeaking impulse, the are established by such peak between anode l6 and the cathode terminates prior to the zero at the regular holding anode 15 which is energized from .a substantially sinusoidal source *cf voltage. Thus when the peakterminatesin winding 54 the lower terminal of winding :55.'is unnre positive than the intermediate terminal 58 so that a current is caused to flow through a -portion of the winding 55 from the voltage across resistor 48". This current is in a direction such as to magnetize the core .in a direction opposite to the magnetization effected by the :current impulses in transformer winding 54. A resistor 62 of the nonlinear type, previously referred to, is connected across the cathode grid .=circuit as a general voltage protective means for the grid circuits.

The grids l0 :and H are preferably connected to the cathode :throughthe capacitors 63 and 64, respectively, in order to make the grid potential follow the "absolute cathode potential during sudden changes in its potential particularly during the commutation period when the valve are functioning as an inverter.

The grid .9, above referred toas an intermediate anode, is connected through a resistor 55 to -an intermediate point of resistors'fiii and 5'! which areconnected in series relation across the anode and cathode of valve '5 and serve to divide the potential gradients between the anode and cathode during no-nconducting "periods.

The general sequence of operation of the ignition and excitation circuits of the electric valvesystem is substantially as follows: The particular ignitor circuit showngenerates a positive peak of current in each half cycle of applied voltage upon discharge of the capacitor 25 through the firing reactor 25' and the firing'transformer 2i The peak .on one-:halfmycle causes current to ifl'OW throughzthe'ignitors +3 and H! of valve'fi :toestab- 'lish a cathode spot :and thecpeak on the following half .cycle'causes'current to flow 'l'80degreesil-ater through-theignitors 13 and M of valve 1. Upon establishment-of the cathode spot, in for example valve '6, the current in the auxiliary anodes l5 starts'to flow'andtherebyImaintainS the cathode spot afterthe ignitor peak has subsided for the desired remaining'portion of thecycle. As soon as the auxiliary anode l6 lisrendered-conductive upon firing of the grid pteak'er transformer 50, the circuit through the secondary winding '52 of the peaker transformerxis completed :and'the peaker voltage is introduced into thexcircuit 'of grids ID and-I! withasteeply risingpositive wave sufficient to :render the grids positive and establish conduction :in'the valve. :Both grids must be made positive to establish conduction.

,.l low if the .cathodeispot jis not formed and the electron emission is insufficient at the desired time :and in the intended :manner and 'the grids were made positive in the absence of "a cathode spot or insufficient electron emission from the cathode, electrons are -drawn from the space below the grids and ionization follows so "that voltage gradients' -rnay become suiiicient'to form a cathode spot on the "side walls or many other undesirable points within the valve envelope.

However, in the illustrated embodiment of my invention, the grids cannot be made positive unless'the auxiliary anode I6 is conducting since in'the absenceof a conducting path in the secondary winding 52 of the peaker transformer, the peaker impulse is not transmitted to the grids and the grids are maintained at a negative potential. i

Whil'elhave shown and described a particular embodiment of my invention, it will be obvious to those :skilled in the art that various changes and modifications may be made without departing from my invention, and I, therefore, aim in the appended claims to cover an such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and :desire ito'secure 'T'loy Letters :Patent of the United States is:

:1. In combination, an electric discharge device of the typ'e employingan ionizable medium and comprising :an anode, a, cathode a control electrode means for initiating electron emission at said cathod'e, an excitation circuit for said control electrode, a source of excitatio-n'voltage, means for establishing a path for electron emission from said cathode, and means including said path of electron emission from said cathode for impressing said source of voltage upon said excitation circuit for determining the instant of conduction between said anode and cathode.

2. In combination, an electron discharge device of the type employing an ionizable medium and comprising an anode, a cathode and a control electrode, means for periodically establishing electron-emission at said cathode, an excitation circuit for said control electrode, a source of excitation voltage, means interposedbetween said cathode and said control electrode for collectin electron emission from said cathode, and electric circuit means including the path of electron emission between said -cathode and said lastmentioned means for introducing source of voltage in said excitation circuit for determining the instant of conduction between said anode .and cathode.

3. lnrcombinatinn, an electron discharge device or the type employing an ionizable medium and comprising an anode, a cathode, an auxiliary electrode and a control electrode, means for establishing a region of electron emission at said cathode, means including said auxiliary electrode for establishing current conduction between said region of electron emission and said auxiliary electrode, and means including a coupling means between said auxiliary electrode and said control electrode for effecting energization of said control electrode only upon initiation of current flow between said auxiliary electrode and said cathode.

4. In combination, a pair of circuits one of which is an alternating current circuit, electric translating apparatus connected between said circuits and comprising an electric discharge device of the type employing an ionizable medium capable of supporting an arc discharge and including an anode, a cathode, an arc initiating member associated with said cathode, an auxiliary electrode associated with said cathode and a control electrode interposed between said anode and said cathode for determining the instant of con-' duction between said anode and cathode, means for controlling said are initiating member to establish an ignition spot on said cathode, means for establishing a current path between said auxiliary electrode and said cathode upon establishment of said ignition spot, an excitation circuit for said control electrode, and means including the current path between said auxiliary electrode and said cathode for introducing a voltage in said excitation circuit of said control electrode to effect conduction between said anode and said cathode.

5. In combination, an alternating current circuit, a direct current circuit, electric translating apparatus connected between said circuits and comprising an electric discharge device of the type employing an ionizable medium capable of supporting an arc discharge and including an anode, a cathode, an ignitor control member associated with said cathode, an auxiliary electrode associated with said cathode, and a control electrode interposed between said anode and said cathode, means connected to said ignitor control member for causing ionization of said medium and for establishing an ignition spot on said cathode, an energizing circuit connected to said auxiliary electrode for establishing current flow therein only upon establishment of said ignition spot on said cathode, an excitation circuit for said control electrode, and means interlocking the excitation circuit of said auxiliary electrode and the excitation circuit of said control electrode for establishing a potential on said control electrode sufiicient to cause conduction between said anode and cathode only upon establishment of current flow in the excitation circuit of said auxiliary electrode. 1

6. In combination, an alternating current circuit, a direct current circuit, electric translating apparatus connected between said circuits and comprising an electric discharge device of the type employing an ionizable medium capable of supporting an arc discharge and including an anode, a cathode, an ignitor control member-associated with said cathode, an auxiliary anode, and a control electrode interposed between said anode and said cathode, a source of excitation, means connected to said source forsupplying current to said ignitor control member for ionizing said medium and for establishing an ignition spot on said cathode, means connected to said source for energizing said auxiliary anode in dependence upon the flow of current between said auxiliary anode and said ignition spot, and means including the current path between said auxiliary anode and said ignition spot for establishing a potential on said control electrode sufficient to cause conduction between said anode and cathode.

'7. In combination, an alternating current circuit, a direct current circuit, electric translating apparatus connected between said circuit and comprising an electric discharge device of the type employing an ionizable medium capable of supporting an arc discharge and including an anode, a cathode, an immersion-ignitor control member associated with said cathode, an auxiliary anode, and a control electrode circuit, an excitation source, means connected to said source for transmitting impulses of energizing current to said immersion-ignitor control member for ionizing said medium and for establishing an ignition spot on said cathode, means connected to said source and including a Winding for producing a voltage of peaked wave form, means connecting said winding in a series circuit with said auxiliary anode, and inductive means interconnecting said series circuit and said control electrode circuit.

8. In combination, an alternating current circuit, a direct current circuit, electric translating apparatus connected between said circuits and comprising an electric discharge device of the type employing an ionizable medium capable of supporting an arc discharge and including an anode, a liquid type cathode, an ignitor control member associated with said cathode, a holding anode and a grid control electrode interposed between said holding anode and said anode, a source of excitation, excitation means connected to said source for effecting the formation of a cathode spot on the surface of said liquid cathode, an excitation circuit for said holding anode, a peaking transformer having a primary winding and a secondary winding, said primary winding being connected to said source of excitation and said secondary winding being connected in series relation with said holding anode, circuit means for connecting said control electrode to said source of excitation, and means for coupling the circuit of said holding anode and the circuit of said control electrode.

9. In combination, an alternating current circuit, a direct'current circuit, electric translating apparatus connected between said circuits and comprising an electric discharge device of the type employing an ionizable medium capable of supporting an arc discharge and including an anode, a cathode, an ignitor control member associated with said cathode, a holding anode, and a grid control electrode interposed between said holding anode and said anode, a source of excitation, means interconnecting said source and saidignitor for effecting the formation of an ignition spot on said cathode, an excitation circuit interconnecting said source and said holding anode, a transformer for producing periodic voltages of'peaked wave form and having a primary winding and a secondary winding, said primary winding being connected to said source of excitation and said secondary winding being connected in series relation with said holding anode for introducing in the circuit of said holding anode periodic voltages of peaked wave form upon conduction of current between said holding anode and said ignition spot, transformer coupling means for interconnecting the circuit of said holding anode and the circuit of said control electrode, means connected to said source of excitation for establishing a negative bias potential, and means for connecting said negative bias potential in the circuit of said control electrode.

10. In combination, an alternating current circuit, a direct current circuit, electric translating apparatus connected between said circuits and comprising an electric discharge device of the type employing an ionizable medium capable of supporting an arc discharge and including an anode, a cathode, an ignitor control member associated with said, cathode, a holding anode and a grid control electrode interposed between said holding anode and said anode, a source of excitation, means interconnecting said source and said ignitor for effecting the formation of an ignition spot on said cathode, an excitation circuit interconnecting said source and said holding anode, means including a peaking transformer having a secondary winding connected in a series circuit with said holding anode, a unidirectional conductive device connected in circuit with said holding anode, a coupling transformer comprising a primary winding and a secondary winding having a pair of end terminals and an intermediate terminal, said primary winding being connected in series relation with the secondary winding of said peaking transformer to said holding anode, the secondary winding of said coupling transformer being connected in series relation with said control electrode, means connected to said source of excitation and including a rectifier and a pair of resistors for producing a unidirectional voltage across said pair of resistors, circuit means for connecting one of said resistors in circuit with said control electrode to impress thereon a negative bias potential, and means connecting the other of said resistors to one of said end terminals and said intermediate terminals for causing a unidirectional current flow between said last mentioned terminals of such a value and polarity as to prevent accumulative saturation on said coupling transformer due to the unidirectional current impulses on said holding anode circuit.

11. In combination, an alternating current circuit, an electric discharge device connected to said alternating current circuit of the type employing an ionizable medium capable of supporting an arc discharge and including an anode, a cathode, an arc initiating member, an auxiliary electrode disposed in the region of said cathodeand a control electrode interposed between said anode and cathode for determining the instant of conduction of said discharge device, means for 10 controlling said are initiating member to establish a source of ionization on said cathode, a source of voltage, means for connecting said source of voltage in circuit with said auxiliary electrode so as to impress unidirectional impulses of voltage upon said auxiliary electrode, an inductive means having a core structure and one winding connected in series relation with said auxiliary electrode and a second winding connected in series relation with said control electrode, and means for reversing the magnetization of said core subsequent to the termination of each of said unidirectional voltage impulses.

12. In combination, an alternating current circuit, a direct current circuit, an electric discharge device connected between said circuits of the type V employing an ionizable medium capable of supporting an arc discharge and including an anode,

I a cathode, an ignitor control member associated with said cathode, an auxiliary anode disposed in the region of said cathode, and a control electrode interposed between said anode and cathode for determining the instant of conduction of said discharge device, means for establishing an ignition spot on said cathode, means including a peaking transformer having a secondary winding connected in series circuit with said holding anode, a unidirectional conductive device connected in circuit with said holding anode, a coupling transformer comprising a primary winding and a secondary winding having a pair of end terminals and an intermediate terminal, said primary winding being connected in series relation with the secondary winding of said peaking transformer to said holding anode, the secondary'winding of said coupling transformer being connected in series relation with said control electrode, a source of unidirectional current, and means for connecting said source to one of said end terminals and to said intermediate terminal for causing a unidirectional current flow between said last mentioned terminals of such a value and polarity as to prevent accumulative saturation in said coupling transformer due to the unidirectional current impulses in said holding anode circuit.

BURNICE D. BEDFORD.

REFERENCES CITED The following references are of record in the file or this patent:

UNITED STATES PATENTS 

